The subject application is related to subject matter disclosed in Japanese Patent Application No. H12-95812 filed on Mar. 30, 2000 in Japan to which the subject application claims priority under Paris Convention and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a thermal type infrared sensor and a method of manufacturing the same. More specially, the present invention relates to a thermal type infrared sensor to detect an infrared signal incident from outside, and a thermal type infrared sensor and method of manufacturing the same which manufacturing is easy, accuracy is high and a SOI substrate is unnecessary.
2. Related Background Art
An infrared sensor can detect objects regardless of day and night, and has a feature in which permeability of an infrared ray for smoke and fog is higher than that of a visible light. Furthermore, because it is possible to obtain temperature information of objects, the infrared sensor is widely used as a surveillance camera and a fire-detecting camera.
As an example of thermal infrared sensor to detect the incident infrared ray, a thermal infrared sensor structure having a pn junction to a detecting portion will be described hereinafter.
The pn junction type of the infrared sensor uses a principle that temperature of the pn junction changes by absorption of the infrared ray, and height of diffusion potential barrier and the number of carriers of the pn junction change by the temperature change.
More specifically, surface of the sensor is provided with an infrared absorber of which temperature changes by irradiation of the infrared ray. The temperature change is, for example, transmitted to a transistor circuit including the pn junction in order to allow a band structure of the pn junction to change. The change of the band structure is read out by voltage change in low current operation. By such a method, the irradiated infrared ray is monitored, and shape of a measuring object and its surface temperature can be detected.
Next, an example of a conventional pn junction type of the infrared sensor will be described by using drawings.
FIG. 1 and FIG. 2 are drawings showing schematic configuration showing an example of a conventional pn junction type of the infrared sensor. FIG. 1 is a plane view, and FIG. 2 is a cross sectional view of an A-Axe2x80x2 line.
In these drawings, numeral 1 denotes a substrate, numeral 2 denotes a detecting portion, numeral 4 denotes a support leg, and numeral 7 denotes a processing circuit section. The substrate under the detecting portion 2 and the support leg 4 is removed by a micro machining and an etching technology in order to form a void. The detecting portion 2 is supported at midair by the support leg 4 extending from the substrate 1.
The detecting portion 2 is formed so that an insulating substrate 12 and insulating films 23 and 32 overlap a detecting layer 21, and an infrared absorption layer 22 is provided on top of the insulated film 32. The infrared ray irradiated to the detecting portion 2 is absorbed by the infrared absorption layer 22. As a result, temperature of the detecting portion 2 goes up. The temperature change is converted into an electric signal by the detecting layer 21, in order to detect the infrared ray. The electric signal converted by the detecting portion 2 is led to the processing circuit section 7. The processing circuit section 7 carries out signal processing in order to perform conversion to an output signal.
In such a conventional structure, in order to improve detecting sensitivity, adiathermancy between the detecting portion 2 and the substrate 1 is important. That is, it is necessary to minimize heat slipping from the detecting portion 2 to the substrate 1. Because of this, the detecting portion 2 is separated from the substrate 1 by the void 6, and a contact point for the substrate 1 is only the support leg 4. The support leg 4 is formed by a material that thermal conductance is low, and it is necessary to restrict the thermal conductance to the substrate by fabricating the support leg 4 thinly and longly by using a micro machining technology, in order to reduce the thermal conductance to the substrate 1.
As a method of forming the void 6 necessary to thermally separate the detecting portion 2 from the substrate 1, an anisotropic etching is used. As an example of etching solutions, there are a potassium hydroxide (KOH) solution and a tetramethylammonium hydroxide (TMAH). The void 6 is fabricated by infusing an etching hole made by the micro machining technology with the above-mentioned etching solution. When fabricating the void 6, it is necessary that the detecting layer 21 be wholly overlapped by the insulating substrate 12 and the insulating films 23 and 32. If the detecting layer 21 contacts the etching hole 51 and the silicon substrate 11, not only the silicon substrate 11 but also the detecting layer 21 are etched when carrying out the anisotropic etching.
In order to easily realize the structure to wholly overlap the above-mentioned detecting portion 2 by the insulating film, there is a method of using a SOI (Silicon-On-Insulator) substrate that the insulating film and a single crystal silicon film are formed on the substrate in order. In the above-mentioned structure, the insulating film plays a role of the insulating substrate 12, and the detecting layer 21 is formed in the SOI portion. After forming the detecting layer 21, the insulating films 23 and 32 are formed on side and top of the detecting portion.
However, in case of using the SOI substrate, thickness of the single crystal silicon substrate and the insulating film has to be determine to take sensitivity of the infrared ray, the anisotropic etching step and so on into account. For example, the structure of a proposed example, i.e. Part of SPIE Conference on Infrared Technology and Applications XXV, vol3698, pp556-564, 1999, uses the pn junction formed to transverse direction. In this example, the single crystal silicon layer has to be thickened in order to enlarge junction area. When the film thickness of the insulating film becomes thick, thermal capacity of the detecting portion increases and high-speed performance of the detecting portion is damaged. On the other hand, when the film thickness of the insulating film becomes thin, the insulating film does not function as an etching block for the detecting layer when carrying out the anisotropic etching.
However, in order to form the detecting portion and the processing circuit on the same SOI substrate, the thicknesses of the single crystal silicon and the insulating layer has to be the same as that of the detecting portion. Therefore, the structure of the semiconductor elements included in the processing circuit section is restricted. For example, in case of using a MOSFET in the processing circuit section, it becomes difficult to avoid influence such as a substrate floating effect or a short channel effect.
Furthermore, because the SOI substrate is more expensive than the silicon substrate, the SOI substrate is also a drawback even in a total cost.
An object of the present invention is to provide a thermal type infrared sensor and a method of manufacturing the same which form a structure that a detecting portion is thermally separated from a substrate without using an expensive substrate such as a SOI substrate and without an etching step, and have a structure capable of arbitrarily designing thicknesses of a single crystal silicon and an insulating film of the detecting portion and the processing circuit section without being restricted to the structure of the substrate. Therefore, it is possible to reduce cost for manufacturing a thermal infrared sensor of pn junction type, thereby improving characteristics of the detecting portion and the processing circuit section.
In order to achieve the foregoing object, a thermal infrared sensor, comprising:
a detecting portion including an absorbing section configured to absorb an incident infrared ray and convert the infrared ray into heat, and a thermoelectric converting section configured to convert temperature change by heat occurred in said absorbing section into an electric signal; and
a processing circuit section including semiconductor elements to process said electric signal obtained by said detecting portion,
wherein said detecting portion is supported in the state of being separated from a semiconductor substrate by at least one of support legs extending to a side face of said detecting portion;
wherein a flat plate-shape void is formed between said detecting portion and said semiconductor substrate; and
wherein said processing circuit section is formed directly on said semiconductor substrate without sandwiching an insulating film.
According to the present invention, because a processing circuit section formed on a semiconductor substrate in order to process a signal detected by a detecting portion is formed so as to contact the semiconductor substrate, it is possible to constitute the processing circuit section without coming under the influence of a substrate structure which occurs in case of using a SOI substrate. Therefore, it is possible to improve characteristics of the processing circuit.
Furthermore, according to the present invention, it is possible to reduce an element capacitance by forming a partial void to the semiconductor element of the processing circuit section, and a substrate floating effect and a short channel effect can be reduced because the element contacts the substrate.
Furthermore, according to the present invention, it is possible to increase the number of the elements by separating the semiconductor elements of the detecting portion by the insulating film, thereby improving detecting sensitivity.
Furthermore, according to the present invention, because an oxide film is formed on internal face of a flat plate-shape void, the oxide film functions as an etching stop layer when fabricating a separated groove for the element separation. Accordingly, it is possible to stably form the element-separated structure, even if the detecting layer is a very thin structure. Furthermore, the oxide film functions as a film for protecting the detecting layer.
Furthermore, according to the present invention, because an optical distance of a depth of the flat plate-shape void is set to substantially xc2xc of the incident infrared ray wavelength, intensity of the infrared ray reflected by a bottom face of the void becomes highest at a location of the detecting portion, thereby improving the detecting sensitivity.
Furthermore, according to the present invention, a flat plate-shape void is fabricated in the substrate by an ESS method, the detecting portion is fabricated in the single crystal silicon layer above the void, the processing circuit section is fabricated on the substrate, and a groove is formed to form the support leg. In this step, it is possible to thermally separate the detecting portion from the substrate without providing the etching step. Furthermore, it is unnecessary to use an expensive substrate such as the SOI substrate, and it is possible to simplify manufacturing steps and to reduce cost.
Thus, according to the present invention, it is possible to provide a thermal type infrared sensor with high efficiency at low cost. Accordingly, industrial advantage of the present invention is very large.
Furthermore, a method of manufacturing a thermal type infrared sensor including a detecting portion configured to absorb an infrared ray and perform thermoelectric conversion, and a processing circuit section including semiconductor elements to process an electric signal obtained by said detecting portion, said detecting portion being supported in the state of being separated from a semiconductor substrate by at least one or more of a support leg extending to a side face of said detecting portion, composing the steps of:
forming a plurality of trenches on the semiconductor substrate;
closing up opening of each of said plurality of trenches in order to change said trenches into a flat plate-shape void, by performing a heat treatment to the semiconductor substrate;
forming said detecting layer in a single crystal silicon layer above said flat plate-shape void;
forming said processing circuit section on said semiconductor substrate; and
forming said support leg by forming a support leg-forming groove of penetrating to said flat plate-shape void.
Here, as a method of forming the flat plate-shape void, there is a method described to a specification applied to Japanese Patent Office, i.e. application number 246582/1999 (H11-246582). This method relates to the ESS (formation of empty space in silicon) method. This method fabricates two-dimensional grooves (trenches) with the same depth at a constant interval. Only upper portions of the grooves are closed up by performing high temperature anneal in hydrogen air, and the lower portions of the grooves are combined with near grooves. As a result, the flat plate-shape void is formed in the silicon substrate, and the flat plate shape of the single crystal silicon is formed on the upper portion of the void.